Semiconductor device

ABSTRACT

A semiconductor device includes a first transistor which includes a first gate electrode below its oxide semiconductor layer and a second gate electrode above its oxide semiconductor layer, and a second transistor which includes a first gate electrode above its oxide semiconductor layer and a second gate electrode below its oxide semiconductor layer and is provided so as to at least partly overlap with the first transistor. In the semiconductor device, a conductive film serving as the second gate electrode of the first transistor and the second gate electrode of the second transistor is shared between the first transistor and the second transistor. Note that the second gate electrode not only controls the threshold voltages (Vth) of the first transistor and the second transistor but also has an effect of reducing interference of an electric field applied from respective first gate electrodes of the first transistor and the second transistor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.

Note that in this specification, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, an electronic device, and the like are all semiconductor devices.

2. Description of the Related Art

A technique for forming a transistor by using a semiconductor film formed over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit (IC) and an image display device (display device). A silicon-based semiconductor film is known as a semiconductor film applicable to the transistor. In recent years, an oxide semiconductor film has attracted attention.

For example, a transistor using an amorphous oxide semiconductor film which contains indium, gallium, and zinc and has an electron carrier concentration of lower than 10¹⁸/cm³ is disclosed (see Patent Document 1).

Since the oxide semiconductor film has a high electron mobility, the operation speed of a transistor using the oxide semiconductor film is significantly higher than that of a transistor using an amorphous silicon film. Further, the transistor using the oxide semiconductor film has an advantage of suppression of equipment investment, because production equipment for the transistor using the amorphous silicon film can be applied thereto with partly improvement.

Further, a memory to which characteristics of a transistor using an oxide semiconductor film is applied has been proposed (see Patent Document 2). Note that an oxide semiconductor film can be formed by a technique for forming a thin film such as a sputtering method. Further, a transistor using an oxide semiconductor film can be manufactured through a process performed at a low temperature lower than or equal to about 350° C. Therefore, there are few limitations on manufacturing a transistor using an oxide semiconductor film so as to overlap with a transistor, which allows reduction in the cell area.

In a transistor using an oxide semiconductor film, carriers may be generated due to an impurity such as hydrogen and oxygen vacancy, and the threshold voltage (Vth) of the transistor may be shifted in a negative direction. Therefore, controlling the threshold voltage (Vth) of the transistor using an oxide semiconductor film by employing a structure of a transistor including gate electrodes above and below its oxide semiconductor film, in which a back gate electrode which forms a pair with a front gate electrode is provided, has been considered (see Patent Document 3).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No. 2006-165528

[Patent Document 2] Japanese Published Patent Application No. 2011-151383

[Patent Document 3] Japanese Published Patent Application No. 2010-283338

SUMMARY OF THE INVENTION

To manufacture a transistor including gate electrodes above and below its oxide semiconductor layer, a front gate electrode (also referred to as a first gate electrode) and a back gate electrode (also referred to as a second gate electrode) are formed, and accordingly the number of steps is increased.

Further, the area per one transistor can be reduced in a semiconductor device by stacking a plurality of transistors each using an oxide semiconductor film; however, in the case where a transistor including gate electrodes above and below its oxide semiconductor layer is provided so as to at least partly overlap with a transistor including gate electrodes above and below its oxide semiconductor layer, a second gate electrode is provided per transistor.

In view of the foregoing, an object of one embodiment of the present invention is to improve the yield of a semiconductor device in which a transistor including gate electrodes above and below its oxide semiconductor layer overlaps with a transistor including gate electrodes above and below its oxide semiconductor layer. Further, an object of one embodiment of the present invention is to suppress an increase in the number of manufacturing steps of the semiconductor device.

One embodiment of the present invention is a semiconductor device including a first transistor including gate electrodes above and below its oxide semiconductor layer and a second transistor which includes gate electrodes above and below its oxide semiconductor layer and is provided so as to at least partly overlap with the first transistor. A conductive film serving as a second gate electrode is shared between the first transistor and the second transistor.

Note that the second gate electrode not only controls the threshold voltages (Vth) of the first transistor and the second transistor but also has an effect of reducing interference of an electric field applied from respective first gate electrodes of the first transistor and the second transistor.

Another embodiment of the present invention is a semiconductor device which includes an insulating film having a planar upper surface provided over a second transistor; a third transistor which has the same structure as a first transistor and is provided over the insulating film; and a fourth transistor which has the same structure as the second transistor and is provided so as to at least partly overlap with the third transistor. A conductive film serving as a second gate electrode is shared between the third transistor and the fourth transistor.

As described above, the first transistor and the second transistor can be configured to be one structure, and the plurality of structures can be provided so as to be stacked. Specifically, the number of the structures which are stacked may be 2 or more and 10 or less, preferably 3 or more and 20 or less, more preferably 5 or more and 50 or less. Needless to say, the number of the structures which are stacked may be set beyond 50.

In the case where the plurality of structures each described above are stacked, the conductive film serving as the second gate electrode is provided per 2 transistors. Accordingly, the number of steps for forming second gate electrodes can be reduced. In other words, one conductive film serving as a second gate electrode can be omitted per structure. Thus, in the case where the plurality of structures are provided so as to be stacked, breaking caused by stress during manufacture of the semiconductor device can be suppressed, so that a semiconductor device including more transistors can be manufactured. In other words, the semiconductor device can be highly integrated and the yield can be improved. Further, even when the plurality of structures are stacked as described above, interference of an electric field can be reduced by the effect of the second gate electrode.

Note that the semiconductor device of one embodiment of the present invention is suitable for a memory. This is because the cell area can be reduced by overlapping a plurality of memory cells.

Another embodiment of the present invention is a semiconductor device including a first transistor provided over an insulating surface and a second transistor provided so as to at least partly overlap with the first transistor. The first transistor includes a first conductive film; a first insulating film provided over the first conductive film; a first oxide semiconductor film provided over the first insulating film; a second conductive film and a third conductive film which are provided so as to be at least partly in contact with the first oxide semiconductor film; a second insulating film provided over the second conductive film, the third conductive film, and the first oxide semiconductor film; and a fourth conductive film provided over the second insulating film. The second transistor includes the fourth conductive film; a third insulating film provided over the fourth conductive film; a second oxide semiconductor film provided over the third insulating film; a fifth conductive film and a sixth conductive film which are provided so as to be at least partly in contact with the second oxide semiconductor film; a fourth insulating film provided over the fifth conductive film, the sixth conductive film, and the second oxide semiconductor film; and a seventh conductive film provided over the fourth insulating film. The first oxide semiconductor film at least partly overlaps with the first conductive film and the fourth conductive film. The second oxide semiconductor film at least partly overlaps with the fourth conductive film and the seventh conductive film. Note that a fifth insulating film having a planar upper surface is preferably provided over the seventh conductive film and the fourth insulating film of the second transistor.

Further, an embodiment of the present invention is a semiconductor device which has a second structure similar to a first structure including the first transistor and the second transistor over the first structure.

In the embodiment, the first conductive film at least partly serves as the first gate electrode of the first transistor. The seventh conductive film at least partly serves as the first gate electrode of the second transistor. The fourth conductive film at least partly serves as the second gate electrode of the first transistor. The fourth conductive film at least partly serves as the second gate electrode of the second transistor.

Another embodiment of the present invention is a semiconductor device including a first conductive film provided over an insulating surface; a first insulating film provided over the first conductive film; a first oxide semiconductor film which is provided over the first insulating film and at least partly overlaps with the first conductive film; a second conductive film and a third conductive film which are provided over the first oxide semiconductor film; a second insulating film provided over the second conductive film, the third conductive film, and the first oxide semiconductor film; a fourth conductive film which is provided over the second insulating film and at least partly overlaps with the second conductive film; a fifth conductive film which is provided over the second insulating film and at least partly overlaps with the first oxide semiconductor film; a third insulating film provided over the second insulating film, the fourth conductive film, and the fifth conductive film; a second oxide semiconductor film which is provided over the third insulating film and at least partly overlaps with the fifth conductive film; a sixth conductive film and a seventh conductive film which are provided over the second oxide semiconductor film; a fourth insulating film provided over the sixth conductive film, the seventh conductive film, and the second oxide semiconductor film; an eighth conductive film which is provided over the fourth insulating film and at least partly overlaps with the sixth conductive film; a ninth conductive film which is provided over the fourth insulating film and at least partly overlaps with the second oxide semiconductor film; a fifth insulating film provided over the fourth insulating film, the eighth conductive film, and the ninth conductive film; and a tenth conductive film provided over the fifth insulating film. Note that an opening reaching the third conductive film is formed in the second insulating film, the third insulating film, the second oxide semiconductor film, the seventh conductive film, the fourth insulating film, and the fifth insulating film. The tenth conductive film is in contact with the third conductive film through the opening. Note that a sixth insulating film having a planar upper surface is preferably provided over the tenth conductive film.

Another embodiment of the present invention is a semiconductor device including a first conductive film provided over an insulating surface; a first insulating film provided over the first conductive film; a first oxide semiconductor film which is provided over the first insulating film and at least partly overlaps with the first conductive film; a second conductive film and a third conductive film which are provided over the first oxide semiconductor film; a second insulating film provided over the second conductive film, the third conductive film, and the first oxide semiconductor film; a fourth conductive film which is provided over the second insulating film and at least partly overlaps with the second conductive film; a fifth conductive film which is provided over the second insulating film and at least partly overlaps with the first oxide semiconductor film; a third insulating film provided over the second insulating film, the fourth conductive film, and the fifth conductive film; a sixth conductive film which is provided over the third insulating film and at least partly overlaps with the fourth conductive film; a seventh conductive film provided over the third insulating film; a second oxide semiconductor film which is provided over the third insulating film, the sixth conductive film, and the seventh conductive film and at least partly overlaps with the fifth conductive film; a fourth insulating film provided over the sixth conductive film, the seventh conductive film, and the second oxide semiconductor film; an eighth conductive film which is provided over the fourth insulating film and at least partly overlaps with the second oxide semiconductor film; a fifth insulating film provided over the eighth conductive film; and a ninth conductive film provided over the fifth insulating film. Note that an opening reaching the third conductive film is formed in the second insulating film, the third insulating film, the second oxide semiconductor film, the seventh conductive film, the fourth insulating film, and the fifth insulating film. The ninth conductive film is in contact with the third conductive film through the opening. Note that a sixth insulating film having a planar upper surface is preferably provided over the ninth conductive film.

A conductive film serving as a second gate electrode is shared between two transistors each including gate electrodes above and below its oxide semiconductor layer, whereby the semiconductor device can be highly integrated and the yield can be improved. Further, it is possible to suppress an increase in the number of manufacturing steps of the semiconductor device

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are a top view and a cross-sectional view, respectively, illustrating an example of a semiconductor device of one embodiment of the present invention;

FIGS. 2A to 2D are cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIGS. 3A to 3C are cross-sectional views illustrating a method for manufacturing the semiconductor device of one embodiment of the present invention;

FIGS. 4A to 4C are cross-sectional views illustrating a method for manufacturing the semiconductor device of one embodiment of the present invention;

FIGS. 5A and 5B are cross-sectional views illustrating a method for manufacturing the semiconductor device of one embodiment of the present invention;

FIGS. 6A and 6B are cross-sectional views illustrating a method for manufacturing the semiconductor device of one embodiment of the present invention;

FIGS. 7A and 7B are a top view and a cross-sectional view, respectively, illustrating an example of a semiconductor device of one embodiment of the present invention;

FIG. 8 is a circuit diagram which corresponds to a semiconductor device of one embodiment of the present invention.

FIGS. 9A to 9D are cross-sectional views illustrating a method for manufacturing a semiconductor device of one embodiment of the present invention;

FIGS. 10A to 10C are cross-sectional views illustrating a method for manufacturing the semiconductor device of one embodiment of the present invention;

FIGS. 11A to 11C are cross-sectional views illustrating a method for manufacturing the semiconductor device of one embodiment of the present invention;

FIGS. 12A and 12B are cross-sectional views illustrating a method for manufacturing the semiconductor device of one embodiment of the present invention;

FIGS. 13A and 13B are cross-sectional views illustrating a method for manufacturing the semiconductor device of one embodiment of the present invention;

FIGS. 14A and 14B are cross-sectional views illustrating a method for manufacturing the semiconductor device of one embodiment of the present invention;

FIG. 15 is a cross-sectional view illustrating a method for manufacturing the semiconductor device of one embodiment of the present invention;

FIGS. 16A and 16B are a top view and a cross-sectional view, respectively, illustrating an example of a semiconductor device of one embodiment of the present invention;

FIGS. 17A and 17B are cross-sectional views illustrating an example of a semiconductor device of one embodiment of the present invention;

FIGS. 18A to 18C are perspective views each illustrating an electronic device of one embodiment of the present invention; and

FIGS. 19A and 19B are a top view and a cross-sectional view, respectively, illustrating a structure of a transistor.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways without departing from the spirit and the scope of the present invention. Therefore, the present invention is not construed as being limited to description of the embodiments. In describing structures of the present invention with reference to the drawings, the same reference numerals are used in common for the same portions in different drawings. Note that the same hatch pattern is applied to similar parts, and the similar parts are not especially denoted by reference numerals in some cases.

In this specification, when one of a source and a drain of a transistor is called a drain, the other is called a source. That is, they are not distinguished depending on the potential level. Therefore, a portion called a source in this specification can be alternatively referred to as a drain.

Note that voltage refers to a potential difference between a predetermined potential and a reference potential (e.g., a ground potential or a source potential) in many cases. Accordingly, a voltage can also be called a potential. Even when a potential is represented as, for example, a potential VH, a potential VDD, or a potential GND, the potential is not exactly equal to the potential VH, the potential VDD, or the potential GND in some cases. Therefore, the potential VH, the potential VDD, and the potential GND can be referred to as a potential close to the potential VH, a potential close to the potential VDD, and a potential close to the potential GND, respectively. Note that “to be grounded” has the same meaning as “to be connected to GND”.

Further, even when the expression “to be connected” is used in this specification, there is a case in which no physical connection is made in an actual circuit and a wiring is just extended.

Note that the ordinal numbers such as “first” and “second” in this specification are used for convenience and do not denote the order of steps or the stacking order of layers. In addition, the ordinal numbers in this specification do not denote particular names which specify the present invention.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of the present invention will be described with reference to FIGS. 1A and 1B, FIGS. 2A to 2D, FIGS. 3A to 3C, FIGS. 4A to 4C, FIGS. 5A and 5B, and FIGS. 6A and 6B.

FIG. 1A is a top view illustrating a semiconductor device of one embodiment of the present invention. FIG. 1B is a cross-sectional view taken along dashed-dotted line A-B in FIG. 1A. In FIG. 1A, a film which is not seen due to overlap, an insulating film, and the like, are omitted for simplicity.

A semiconductor device illustrated in FIG. 1B includes a first conductive film 104 a provided over a first insulating film 102 which is provided over a substrate 101; a second insulating film 112 a provided over the first conductive film 104 a and the first insulating film 102; a first oxide semiconductor film 106 a which is provided over the second insulating film 112 a and at least partly overlaps with the first conductive film 104 a; a second conductive film 116 a provided over the first oxide semiconductor film 106 a; a third conductive film 126 a provided over the first oxide semiconductor film 106 a; a third insulating film 110 a provided over the second conductive film 116 a, the third conductive film 126 a, and the first oxide semiconductor film 106 a; a fourth conductive film 114 which is provided over the third insulating film 110 a and at least partly overlaps with the first oxide semiconductor film 106 a and the first conductive film 104 a; a fourth insulating film 120 a which is provided over the third insulating film 110 a and whose upper surface is level with that of the fourth conductive film 114; a fifth insulating film 110 b provided over the fourth conductive film 114 and the fourth insulating film 120 a; a second oxide semiconductor film 106 b which is provided over the fifth insulating film 110 b and at least partly overlaps with the fourth conductive film 114; a fifth conductive film 116 b which is provided over the second oxide semiconductor film 106 b and which at least partly overlap with the second conductive film 116 a; a sixth conductive film 126 b which is provided over the second oxide semiconductor film 106 b and which at least partly overlap with the third conductive film 126 a; a sixth insulating film 112 b provided over the fifth conductive film 116 b, the sixth conductive film 126 b, and the second oxide semiconductor film 106 b; and a seventh conductive film 104 b which is provided over the sixth insulating film 112 b and at least partly overlaps with the second oxide semiconductor film 106 b and the fourth conductive film 114. Note that a seventh insulating film 120 b having a planar upper surface is preferably provided over the seventh conductive film 104 b and the sixth insulating film 112 b.

Note that in FIG. 1B, the first conductive film 104 a, the fourth conductive film 114, and the seventh conductive film 104 b do not overlap with any of the second conductive film 116 a, the third conductive film 126 a, the fifth conductive film 116 b, and the sixth conductive film 126 b; however, embodiments of the present invention are not limited thereto. For example, the first conductive film 104 a, the fourth conductive film 114, and the seventh conductive film 104 b may at least partly overlap with the second conductive film 116 a, the third conductive film 126 a, the fifth conductive film 116 b, and the sixth conductive film 126 b.

There is no particular limitation on the substrate 101 as long as it has heat resistance enough to withstand at least heat treatment performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, or a sapphire substrate may be used as the substrate 101. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, a silicon-on-insulator (SOI) substrate, or the like may be used as the substrate 101. Still alternatively, any of these substrates further provided with a semiconductor element may be used as the substrate 101.

In the case of using a large glass substrate such as the fifth generation (1000 mm×1200 mm or 1300 mm×1500 mm); the sixth generation (1500 mm×1800 mm); the seventh generation (1870 mm×2200 mm); the eighth generation (2200 mm×2500 mm); the ninth generation (2400 mm×2800 mm); or the tenth generation (2880 mm×3130 mm) as the substrate 101, in some cases, microfabrication is difficult due to the shrinkage of the substrate 101, which is caused by heat treatment or the like in a manufacturing process of the semiconductor device. Accordingly, in the case of using a large glass substrate described above as the substrate 101, a substrate with small shrinkage is preferably used. For example, a large glass substrate whose shrinkage by heat treatment for one hour at 400° C., preferably 450° C., more preferably 500° C. is less than or equal to 10 ppm, preferably less than or equal to 5 ppm, more preferably less than or equal to 3 ppm, may be used as the substrate 101.

Further, a flexible substrate may be used as the substrate 101. Note that as a method for forming a transistor over a flexible substrate, there is also a method in which, after a transistor is formed over a non-flexible substrate, the transistor is separated and transferred to the substrate 101 which is a flexible substrate. In that case, a separation layer is preferably provided between the non-flexible substrate and the transistor.

The first insulating film 102 is provided in order that an impurity attributed to the substrate 101 does not adversely affect the first oxide semiconductor film 106 a. In contrast, in the case where the substrate 101 does not contain an impurity, the first insulating film 102 is not necessarily provided.

The first insulating film 102 may be formed of a single layer or a stacked layer of an insulating film(s) containing any of aluminum oxide, aluminum nitride, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide.

In this specification, silicon oxynitride refers to a substance that contains more oxygen than nitrogen. For example, silicon oxynitride includes oxygen, nitrogen, silicon, and hydrogen at concentrations ranging from greater than or equal to 50 atomic % and less than or equal to 70 atomic %, greater than or equal to 0.5 atomic % and less than or equal to 15 atomic %, greater than or equal to 25 atomic % and less than or equal to 35 atomic %, and greater than or equal to 0 atomic % and less than or equal to 10 atomic %, respectively. In addition, silicon nitride oxide refers to a substance that contains a larger amount of nitrogen than that of oxygen. For example, silicon nitride oxide contains oxygen, nitrogen, silicon, and hydrogen at concentrations ranging from greater than or equal to 5 atomic % and less than or equal to 30 atomic %, greater than or equal to 20 atomic % and less than or equal to 55 atomic %, greater than or equal to 25 atomic % and less than or equal to 35 atomic %, and greater than or equal to 10 atomic % and less than or equal to 25 atomic %, respectively. Note that the above ranges are ranges for cases where measurement is performed using Rutherford backscattering spectrometry (RBS) and hydrogen forward scattering spectrometry (HFS). Moreover, the total of the percentages of the constituent elements does not exceed 100 atomic %.

The first conductive film 104 a at least partly serves as a first gate electrode in a transistor using the first oxide semiconductor film 106 a. For example, the first conductive film 104 a may be formed of a single layer or a stacked layer of a simple substance selected from Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ag, Ta, and W; a nitride containing one or more kinds of the above substances; an oxide containing one or more kinds of the above substances; or an alloy containing one or more kinds of the above substances. Alternatively, an oxide or an oxynitride which contains at least In and Zn may be used. For example, an In—Ga—Zn—O—N compound may be used.

The second insulating film 112 a at least partly serves as a gate insulating film. For example, the second insulating film 112 a may be formed of a single layer or a stacked layer of an insulating film(s) containing any of aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide.

The first oxide semiconductor film 106 a includes a channel region in a region which overlaps with the first conductive film 104 a. Note that the channel region is a region in which a channel is formed by the electric field of a gate electrode. When the channel is formed, current flows by application of voltage between a source and a drain.

The first oxide semiconductor film 106 a may be formed using an In-M-Zn—O compound, for example. Here, a metal element M is an element whose bond energy with oxygen is higher than that of In and that of Zn. Alternatively, the metal element M is an element which has a function of suppressing desorption of oxygen from the In-M-Zn—O compound. Owing to the effect of the metal element M, generation of oxygen vacancies in the oxide semiconductor film is suppressed. Therefore, change in characteristics of the transistor, which is caused by oxygen vacancies, can be reduced; accordingly, a highly reliable transistor can be obtained.

The metal element M may be, specifically, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Nb, Mo, Sn, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, or W, and is preferably Al, Ti, Ga, Y, Zr, Ce, or Hf. As the metal element M, one or two or more kinds of the above elements may be selected. Further, Si or Ge may be used instead of the metal element M.

The first oxide semiconductor film 106 a is in a single crystal state, a polycrystalline (also referred to as polycrystal) state, an amorphous state, or the like.

The first oxide semiconductor film 106 a is preferably a CAAC-OS (c-axis aligned crystalline oxide semiconductor) film.

The CAAC-OS film is not completely single crystal nor completely amorphous. The CAAC-OS film is an oxide semiconductor film having a crystal-amorphous mixed structure in which an amorphous phase includes a crystal portion and an amorphous portion. Note that in most cases, the crystal portion fits inside a cube whose one side is less than 100 nm. From an observation image obtained with a transmission electron microscope (TEM), a boundary between an amorphous portion and a crystal portion in the CAAC-OS film is not clear. Further, with the TEM, a grain boundary in the CAAC-OS film is not found. Thus, in the CAAC-OS film, a reduction in carrier mobility, due to the grain boundary, is suppressed.

In each of the crystal portions included in the CAAC-OS film, a c-axis is aligned in a direction perpendicular to a surface where the CAAC-OS film is formed or a surface of the CAAC-OS film, triangular or hexagonal atomic arrangement which is seen from the direction perpendicular to the a-b plane is formed, and metal atoms are arranged in a layered manner or metal atoms and oxygen atoms are arranged in a layered manner when seen from the direction perpendicular to the c-axis. Note that, among crystal portions, the directions of the a-axis and the b-axis of one crystal portion may be different from those of another crystal portion. In this specification, a simple term “perpendicular” includes a range from 85° to 95°.

In the CAAC-OS film, distribution of crystal portions is not necessarily uniform. For example, in the formation process of the CAAC-OS film, in the case where crystal growth starts from a surface side of the first oxide semiconductor film 106 a, the proportion of crystal portions in the vicinity of the surface of the first oxide semiconductor film 106 a is higher than that in the vicinity of the surface where the first oxide semiconductor film 106 a is formed in some cases. Further, when an impurity is added to the CAAC-OS film, the crystal portion in a region to which the impurity is added becomes amorphous in some cases.

Since the c-axes of the crystal portions included in the CAAC-OS film are aligned in the direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, the directions of the c-axes may be different from each other depending on the shape of the CAAC-OS film (the cross-sectional shape of the surface where the CAAC-OS film is formed or the cross-sectional shape of the surface of the CAAC-OS film). Note that when the CAAC-OS film is formed, the direction of c-axis of the crystal portion is the direction parallel to a normal vector of the surface where the CAAC-OS film is formed or a normal vector of the surface of the CAAC-OS film. The crystal portion is formed by film formation or by performing treatment for crystallization such as heat treatment after film formation.

Further, change in characteristics of a transistor using the CAAC-OS film, due to irradiation with visible light or ultraviolet light is small. Thus, the transistor has high reliability.

Note that the oxide semiconductor film (here, each of the first oxide semiconductor film 106 a and the second oxide semiconductor film 106 b) is preferably an oxide semiconductor film which is highly purified and hardly contains impurities such as copper, aluminum, and chlorine. In a process of forming the transistor, a process in which there is not a possibility that the impurities are mixed into the oxide semiconductor film or attached to surfaces of the oxide semiconductor film is preferably selected as appropriate. In the case where the impurities are attached to the surfaces of the oxide semiconductor film, exposure to oxalic acid or dilute hydrofluoric acid, or plasma treatment (N₂O plasma treatment or the like) is preferably performed to remove the impurities attached to the surfaces of the oxide semiconductor film. Specifically, the copper concentration in the oxide semiconductor film is 1×10¹⁸ atoms/cm³ or less, preferably 1×10¹⁷ atoms/cm³ or less. The aluminum concentration in the oxide semiconductor film is 1×10¹⁸ atoms/cm³ or less. The chlorine concentration in the oxide semiconductor film is 2×10¹⁸ atoms/cm³ or less.

In addition, it is preferable that the oxide semiconductor layer contain oxygen in a proportion higher than that in the stoichiometric composition to be supersaturated shortly after the oxide semiconductor layer is deposited. For example, in the case of depositing the oxide semiconductor film by a sputtering method, deposition is preferably performed under conditions where the proportion of oxygen contained in a deposition gas is high, in particular, in an oxygen atmosphere. When deposition is performed under conditions where the proportion of oxygen contained in a deposition gas is high, in particular, in an oxygen atmosphere, Zn is prevented from being released from the oxide semiconductor film even when the deposition temperature is higher than or equal to 300° C.

Here, the oxide semiconductor film is preferably highly purified by sufficient removal of impurities such as hydrogen therefrom. Further, the oxide semiconductor film is preferably supersaturated with oxygen by sufficient supply of oxygen. Specifically, the hydrogen concentration in the oxide semiconductor film is 5×10²⁰ atoms/cm³ or less, preferably 5×10¹⁹ atoms/cm³ or less, more preferably 5×10¹⁸ atoms/cm³ or less, still more preferably 5×10¹⁷ atoms/cm³ or less. Note that the hydrogen concentration in the oxide semiconductor film is measured by secondary ion mass spectrometry (SIMS). Further, an insulating film (e.g. a SiOx film) containing excess oxygen is provided so as to be in contact with and thus cover the oxide semiconductor film so that the oxide semiconductor film can be supersaturated with oxygen by sufficient supply of oxygen.

Further, the hydrogen concentration in the insulating film containing excess oxygen is also an important factor because it affects the characteristics of the transistor.

An influence of the hydrogen concentration in the insulating film containing excess oxygen on the characteristics of the transistor will be described below.

First, hydrogen was intentionally added into the insulating film containing excess oxygen, and the hydrogen concentration was measured by SIMS.

A method for forming samples will be described below.

First, a glass substrate was prepared, and a silicon oxide film was formed over the glass substrate to a thickness of 300 nm by a sputtering method.

The silicon oxide film was formed under the following conditions: a quartz target was used; the pressure was 0.4 Pa; the power was 1.5 kW (13.56 MHz); and the substrate temperature in the film formation was 100° C.

Four kinds of samples were prepared. Note that respective conditions for forming the samples were similar to one another except for the flow rate ratio of the deposition gas for the silicon oxide film, oxygen gas (O₂), a deuterium gas (D₂), and an argon gas (Ar).

Table 1 shows names of the samples; the flow rate ratios of deposition gases used for deposition of silicon oxide films; the D (deuterium atom) concentrations at depths of 30 nm in the silicon oxide films; and the H (hydrogen atom) concentrations at depths of 30 nm in the silicon oxide films. Note that as for the proportions of D₂ (D₂/(O₂+Ar+D₂)) in the deposition gases for the samples, the proportion of D₂ in Sample 1 was 0 vol %, the proportion of D₂ in Sample 2 was 0.005 vol %, the proportion of D₂ in Sample 3 was 0.50 vol %, and the proportion of D₂ in Sample 4 was 2.50 vol %.

TABLE 1 Concentration Concentration Name of O₂ Ar D₂ Proportion of D of H Sample [sccm] [sccm] [sccm] of D₂ [atoms/cm³] [atoms/cm³] Sample 1 25 25 0  0% 5.1E+15 6.4E+19 Sample 2 25 24.9975 0.0025 0.005%  1.6E+19 1.4E+20 Sample 3 25 24.75 0.25 0.5% 5.6E+20 7.2E+19 Sample 4 25 23.75 1.25 2.5% 7.2E+20 1.9E+19

Table 1 shows that the higher the proportion of D₂ in the deposition gases is, the higher the D concentration in the silicon oxide film is.

Next, transistors were formed using Samples 1 to 4 shown in Table 1.

FIG. 19A is a top view illustrating each of transistors which were objects to be measured. FIG. 19B is a cross-sectional view taken along dashed-dotted line A-B in FIG. 19A. Note that for simplicity, a protective insulating film 2118, a gate insulating film 2112, an insulating film 2102, and the like are omitted in FIG. 19A.

The transistor illustrated in FIG. 19B includes a substrate 2100; the insulating film 2102 which contains excess oxygen and is provided over the substrate 2100; an oxide semiconductor film 2106 provided over the insulating film 2102; a pair of electrodes 2116 provided over the oxide semiconductor film 2106; the gate insulating film 2112 provided so as to cover the oxide semiconductor film 2106 and the pair of electrodes 2116; a gate electrode 2104 provided so as to overlap with the oxide semiconductor film 2106 with the gate insulating film 2112 interposed therebetween; and the protective insulating film 2118 provided over the gate electrode 2104 and the gate insulating film 2112.

As the insulating film 2102, any of Samples 1 to 4 shown in Table 1 was used. The thickness of the insulating film 2102 was 300 nm.

As the substrate 2100, a glass substrate was used. As the oxide semiconductor film 2106, a 20 nm-thick IGZO film (formed using a target of In:Ga:Zn=1:1:1 [atomic ratio]) was used. As the pair of electrodes 2116, a 100 nm-thick tungsten film was used. As the gate insulating film 2112, a 30 nm-thick silicon oxynitride film was used. As the gate electrode 2104, a film of a 15 nm-thick tantalum nitride film and a 135 nm-thick tungsten film, which was stacked in this order from the gate insulating film 2112 side, was used. As the protective insulating film 2118, a 300 nm-thick silicon oxynitride film was used.

A BT stress test was performed on the transistors having the above-described structure. Note that for the measurement, the transistors each of which had a channel length (L) of 10 μm and a channel width (W) of 10 μm and in each of which the length (Lov) of a portion where the gate electrode 2104 overlaps with each of the pair of electrodes 2116 in the channel length direction was 1 μm (2 μm in total) were used. A method for the BT stress test will be described below.

First, the drain current (Id) was measured under conditions that the substrate temperature was 25° C., the drain voltage (V_(d)) of the transistor was 3 V, and the gate voltage (V_(g)) of the transistor was swept from −6 V to 6 V. The characteristics of the transistor measured in this manner are called “characteristics of the transistor before the BT stress test”, which are represented by voltage relative to the source potential.

Next, V_(d) was set to 0.1 V, V_(g) was set to −6 V, and the substrate temperature was set to 150° C., under which the transistors were kept for one hour.

Next, the drain current (Id) was measured under conditions that the substrate temperature was 25° C., the drain voltage (V_(d)) of the transistor was 3 V, and the gate voltage (V_(g)) of the transistor was swept from −6 V to 6 V. The characteristics of the transistor measured in this manner are called “characteristics of the transistor after the BT stress test”.

Table 2 shows threshold voltage (V_(th)) and field effect mobility (μ_(FE)) before and after the BT stress test. Note that names of Samples shown in Table 2 corresponds to the names of Samples shown in Table 1; Table 1 can be referred to for the conditions of the insulating film 2102.

TABLE 2 Before BT stress test After BT stress test Name of Vth μ_(FE) Vth μ_(FE) Samples [V] [cm²/Vs] [V] [cm²/Vs] Sample 1 0.94 8.6 1.17 7.8 Sample 2 0.82 8.6 1.03 8.2 Sample 3 0.89 8.8 1.05 7.8 Sample 4 0.71 8.7 0.43 2.5

According to Table 2, it was found that in Sample 4, the field effect mobility (μ_(FE)) was drastically decreased as a result of the BT stress test.

Further, characteristics of respective transistors having smaller L were measured. As a result, variation in threshold voltage (Vth) in a negative direction was larger in Sample 4 than in the other Samples.

As described above, it was found that abnormality of characteristics of the transistor is caused in the case where the D concentration in the silicon oxide film was 7.2×10²⁰ atoms/cm³ in the transistor having a structure in which the silicon oxide film was in contact with the oxide semiconductor film.

As described above, in the case where the hydrogen concentration in the insulating film containing excess oxygen is greater than or equal to 7.4×10²⁰ atoms/cm³ (greater than or equal to the total concentration of the D concentration of 7.2×10²⁰ atoms/cm³+the H concentration of 1.9×10¹⁹ atoms/cm³), it is found that variation in characteristics of the transistor is increased and a channel length dependence of the transistor is increased. Further, the characteristics of the transistor are significantly deteriorated by the BT stress test. Therefore, the hydrogen concentration in the insulating film containing excess oxygen is preferably less than 7.4×10²⁰ atoms/cm³. Specifically, the hydrogen concentration in the oxide semiconductor film is preferably less than or equal to 5×10¹⁹ atoms/cm³, and the hydrogen concentration in the insulating film containing excess oxygen is preferably less than 7.4×10²⁰ atoms/cm³.

Further, a blocking layer (e.g. an AlOx film) which suppress release of oxygen from the oxide semiconductor film is preferably provided outside the insulating film containing excess oxygen so as to cover the oxide semiconductor film.

The insulating layer containing excessive oxygen or the blocking layer is provided to cover the oxide semiconductor layer, whereby the oxide semiconductor layer can contain oxygen in a proportion which is substantially the same as that in the stoichiometric composition, or in a proportion higher than that in the stoichiometric composition i.e., the oxide semiconductor layer can be supersaturated with oxygen. For example, in the case where the oxide semiconductor film is an IGZO film in which the stoichiometric composition is In:Ga:Zn:O=1:1:1:4 [atomic ratio], the atomic ratio of oxygen contained in the IGZO film is larger than 4.

The oxide semiconductor film which contains oxygen sufficiently and is highly purified has a band gap of about 2.8 eV to 3.2 eV, has extremely few minority carriers having a density of about 1×10⁻⁹/cm³. Majority carriers come only from a source of the transistor. Accordingly, in the transistor using the oxide semiconductor film, an avalanche breakdown does not occur.

Further, in the transistor using the oxide semiconductor film, the channel region of the FET is completely depleted by the electric field of the gate electrode; therefore, for example, the off-state current when the channel length is 3 μm and the channel width is 1 μm can be suppressed to be less than or equal to 1×10⁻²³ A at 85° C. to 95° C. and to be less than or equal to 1×10⁻²⁵ A at a room temperature.

The second conductive film 116 a and the third conductive film 126 a at least partly serve as a source electrode and a drain electrode of the transistor using the first oxide semiconductor film 106 a. As the second conductive film 116 a and the third conductive film 126 a, a film selected from the conductive films given as examples of the first conductive film 104 a may be used.

The third insulating film 110 a at least partly serves as a gate insulating film. As the third insulating film 110 a, a film selected from the insulating films given as examples of the second insulating film 112 a may be used.

The fourth conductive film 114 serves as a second gate electrode in the transistor using the first oxide semiconductor film 106 a. As the fourth conductive film 114, a film selected from the conductive films given as examples of the first conductive film 104 a may be used.

Note that formation of a channel in the first oxide semiconductor film 106 a depends on the balance between voltage applied to the fourth conductive film 114 and voltage applied to the first conductive film 104 a. For example, when negative voltage is applied to the fourth conductive film 114, even when voltage corresponding to the threshold voltage (Vth) of the transistor is applied to the first conductive film 104 a, a channel is not formed in the first oxide semiconductor film 106 a. In this case, when voltage which is higher than the threshold voltage (Vth) of the transistor by the negative voltage applied to the fourth conductive film 114 is applied to the first conductive film 104 a, a channel is formed. The same can be said to the case where the fourth conductive film 114 and the first conductive film 104 a are replaced with each other. Needless to say, the balance between voltage applied to the first gate electrode and voltage applied to the second gate electrode varies depending on the thickness or the dielectric constant of the gate insulating film.

Accordingly, in the case where the transistor has normally-on characteristics (the threshold voltage Vth is negative), negative voltage is sufficiently applied to the first conductive film 104 a or the fourth conductive film 114 so that the threshold voltage can be positive, whereby the transistor can have normally-off characteristics (the threshold voltage Vth is positive). It is preferable that the threshold voltage Vth of the transistor be controlled by the fourth conductive film 114.

The fourth insulating film 120 a serves as a planarization film. Accordingly, the films provided above the fourth insulating film 120 a are less likely to be affected by unevenness generated by the films provided below the fourth insulating film 120 a. Thus, a decrease in withstand voltage, disconnection, or the like, which is caused by the unevenness, can be prevented. In other words, the fourth insulating film 120 a is an insulating film having a function of reducing the unevenness of the semiconductor device. Note that in the case where unevenness generated by the films provided below the fourth insulating film 120 a is minor, the fourth insulating film 120 a is not necessarily provided.

The fourth insulating film 120 a may be formed of a single layer or a stacked layer of an insulating film(s) containing any of aluminum oxide, aluminum nitride, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide.

The fourth insulating film 120 a may also be formed using an organic resin such as acrylic, epoxy, or polyimide. An organic resin can be formed by a spin coating method or the like, and thus a relatively planar upper surface is easily obtained.

The fifth insulating film 110 b at least partly serves as a gate insulating film. As the fifth insulating film 110 b, a film selected from the insulating films given as examples of the second insulating film 112 a may be used.

The second oxide semiconductor film 106 b includes a channel region in a region which overlaps with the seventh conductive film 104 b. As the second oxide semiconductor film 106 b, a film selected from the compound films given as examples of the first oxide semiconductor film 106 a may be used.

The fifth conductive film 116 b and the sixth conductive film 126 b at least partly serve as a source electrode and a drain electrode in a transistor using the second oxide semiconductor film 106 b. As the fifth conductive film 116 b and the sixth conductive film 126 b, a film selected from the conductive films given as examples of the first conductive film 104 a may be used.

The sixth insulating film 112 b serves as a gate insulating film. As the sixth insulating film 112 b, a film selected from the insulating films given as examples of the second insulating film 112 a may be used.

The seventh conductive film 104 b at least partly serves as a first gate electrode in the transistor using the second oxide semiconductor film 106 b. As the seventh conductive film 104 b, a film selected from the conductive films given as examples of the first conductive film 104 a may be used.

Note that formation of a channel in the second oxide semiconductor film 106 b depends on the balance between voltage applied to the fourth conductive film 114 and voltage applied to the seventh conductive film 104 b. For example, when negative voltage is applied to the fourth conductive film 114, even when voltage corresponding to the threshold voltage Vth of the transistor is applied to the seventh conductive film 104 b, a channel is not formed in the second oxide semiconductor film 106 b. In this case, when voltage which is higher than the threshold voltage Vth of the transistor by the negative voltage applied to the fourth conductive film 114 is applied to the seventh conductive film 104 b, a channel is formed. The same can be said to the case where the fourth conductive film 114 and the seventh conductive film 104 b are replaced with each other.

Accordingly, in the case where the transistor has normally-on characteristics, negative voltage is sufficiently applied to the seventh conductive film 104 b or the fourth conductive film 114 so that the threshold voltage Vth can be positive, whereby the transistor can have normally-off characteristics.

Thus, the threshold voltage Vth of the first oxide semiconductor film 106 a and the second oxide semiconductor film 106 b can be controlled by the fourth conductive film 114. In other words, one second gate electrode (here, the fourth conductive film 114) serves as the second gate electrode both for the transistor using the first oxide semiconductor film 106 a and the transistor using the second oxide semiconductor film 106 b. Accordingly, the second gate electrode can be shared between the transistors of the semiconductor device, each including gate electrodes above and below its oxide semiconductor layer.

Note that the seventh insulating film 120 b serves as a planarization film. Accordingly, films provided above the seventh insulating film 120 b are less likely to be affected by unevenness generated by the films provided below the seventh insulating film 120 b. Thus, a decrease in withstand voltage, disconnection, or the like, which is caused by the unevenness, can be prevented. In other words, the seventh insulating film 120 b is an insulating film having a function of reducing the unevenness of the semiconductor device.

The seventh insulating film 120 b enables a stack of the plurality of structures of the semiconductor device described in this embodiment (here, the structure including the components from the first insulating film 102 to the seventh insulating film 120 b). Specifically, the number of the structures which are stacked is 2 or more and 10 or less, preferably 3 or more and 20 or less, more preferably 5 or more and 50 or less. Needless to say, the number of the structures which are stacked may be set beyond 50.

In the case where the plurality of structures of the semiconductor device described in this embodiment are stacked, one conductive film serving as the second gate electrode is provided per two transistors. Accordingly, the number of steps for forming second gate electrodes can be reduced. In other words, one conductive film serving as a second gate electrode can be omitted per structure. Thus, in the case where the plurality of structures are provided so as to be stacked, breaking due to stress during manufacture of the semiconductor device can be suppressed, so that a semiconductor device including more transistors can be manufactured. In other words, the semiconductor device can be highly integrated and the yield can be improved.

Next, a method for manufacturing the semiconductor device illustrated in FIG. 1B will be described with reference to FIGS. 2A to 2D, FIGS. 3A to 3C, FIGS. 4A to 4C, FIGS. 5A and 5B, and FIGS. 6A and 6B.

First, the substrate 101 is prepared.

Next, the first insulating film 102 is formed over the substrate 101 (see FIG. 2A). The first insulating film 102 can be formed by a sputtering method, a chemical vapor deposition (CVD) method, a molecular beam epitaxy (MBE) method, an atomic layer deposition (ALD) method, or a pulsed laser deposition (PLD) method, for example.

Next, a conductive film is formed and processed to form the first conductive film 104 a (see FIG. 2B). The conductive film may be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method, for example.

Note that the simple term “processed” means in this specification that the film is formed into a desired shape using a resist mask formed by a photolithography process, for example.

Next, the second insulating film 112 a is formed (see FIG. 2C). The second insulating film 112 a can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method, for example.

Next, an oxide semiconductor film is formed and then processed to form the first oxide semiconductor film 106 a (see FIG. 2D). The oxide semiconductor film may be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method, for example.

Next, a conductive film is formed and then processed to form the second conductive film 116 a and the third conductive film 126 a (see FIG. 3A). The conductive film may be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method, for example.

Then, the third insulating film 110 a is formed (see FIG. 3B). The third insulating film 110 a can be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method, for example.

Next, a conductive film is formed and then processed to form a conductive film 124 (see FIG. 3C). The conductive film may be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method, for example.

Next, an insulating film 130 a is formed (see FIG. 4A). The insulating film 130 a may be formed by a spin coating method, a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method, for example.

Next, the insulating film 130 a and the conductive film 124 are removed from the upper surfaces so that they have planarity, so that the fourth insulating film 120 a and the fourth conductive film 114 whose upper surfaces are level with each other are formed (see FIG. 4B). Note that the insulating film 130 a and the conductive film 124 are formed to have a sufficient thickness in advance, which makes it easy to form the fourth insulating film 120 a and the fourth conductive film 114 whose upper surfaces are level with each other.

As a method for removing the insulating film 130 a and the conductive film 124 from the upper surfaces so that they have planarity, a chemical mechanical polishing (CMP) method may be used, for example.

Then, the fifth insulating film 110 b is formed (see FIG. 4C). The fifth insulating film 110 b may be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method, for example.

Next, an oxide semiconductor film is formed and then processed to form the second oxide semiconductor film 106 b (see FIG. 5A). The oxide semiconductor film may be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method, for example.

Next, a conductive film is formed and then processed to form the fifth conductive film 116 b and the sixth conductive film 126 b (see FIG. 5B). The conductive film may be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method, for example.

Next, the sixth insulating film 112 b is formed (see FIG. 6A). The sixth insulating film 112 b may be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method, for example.

Next, a conductive film is formed and then processed to form the seventh conductive film 104 b (see FIG. 6B). The conductive film may be formed by a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method, for example.

Next, the seventh insulating film 120 b is formed. In this manner, the semiconductor device illustrated in FIG. 1B is manufactured. The seventh insulating film 120 b may be formed by a spin coating method, a sputtering method, a CVD method, an MBE method, an ALD method, or a PLD method, for example.

Note that in order to planarize an upper surface of the seventh insulating film 120 b, CMP or the like may be performed thereon.

Note that in a step before or after any of the respective processings for forming the first oxide semiconductor film 106 a and the second oxide semiconductor film 106 b, heat treatment may be performed at a temperature higher than or equal to 100° C. and lower than or equal to 450° C., preferably higher than or equal to 150° C. and lower than or equal to 400° C., more preferably higher than or equal to 200° C. and lower than or equal to 350° C. Further, when the oxide semiconductor film serving as the first oxide semiconductor film 106 a or the second oxide semiconductor film 106 b is formed, the substrate temperature may be higher than or equal to 100° C. and lower than or equal to 450° C., preferably higher than or equal to 150° C. and lower than or equal to 400° C., more preferably higher than or equal to 200° C. and lower than or equal to 350° C. By the heat treatment and/or the formation method, hydrogen, halogen, a metal element which is not a main component, an oxygen vacancy, or the like in the first oxide semiconductor film 106 a or the second oxide semiconductor film 106 b can be reduced, so that a semiconductor device having excellent characteristics and high reliability can be manufactured.

Since the semiconductor device manufactured as described above has a planar upper surface, a plurality of structures similar to the structure of the semiconductor device can be provided so as to be stacked. For example, when a structure including the components from the first insulating film 102 to the seventh insulating film 120 b in FIG. 17A is referred to as A structure (this structure corresponds to the structure in FIG. 1B), the plurality of A structures may be stacked as illustrated in FIG. 17B. Accordingly, the degree of integration of the semiconductor device can be increased. Further, the number of the layers for forming the second gate electrodes in the transistors each including gate electrodes above and below the oxide semiconductor layer can be made small, and thus stress can be prevented from being increased even when the plurality of structures similar to the structure of the semiconductor device are stacked. In other words, even when the degree of integration of the semiconductor device is increased, the yield of the semiconductor device is not decreased.

This embodiment can be implemented in appropriate combination with any of the other embodiments.

Embodiment 2

In this embodiment, a semiconductor device having a structure different from that of the semiconductor device in Embodiment 1 will be described with reference to FIGS. 7A and 7B, FIG. 8, FIGS. 9A to 9D, FIGS. 10A to 10C, FIGS. 11A to 11C, FIGS. 12A and 12B, FIGS. 13A and 13B, FIGS. 14A and 14B, and FIG. 15.

FIG. 7A is a top view of a semiconductor device of one embodiment of the present invention. Note that FIG. 7B is a cross-sectional view taken along dashed-dotted line A-B in FIG. 7A. In FIG. 7A, a film which is not seen due to overlap, an insulating film, and the like, are omitted for simplicity.

The semiconductor device illustrated in FIG. 7B includes a first conductive film 204 a provided over a first insulating film 202 which is provided over a substrate 201; a second insulating film 212 a provided over the first conductive film 204 a and the first insulating film 202; a first oxide semiconductor film 206 a which is provided over the second insulating film 212 a and at least partly overlaps with the first conductive film 204 a; a second conductive film 216 a and a third conductive film 226 a which are provided over the first oxide semiconductor film 206 a; a third insulating film 210 a provided over the second conductive film 216 a, the third conductive film 226 a, and the first oxide semiconductor film 206 a; a fourth conductive film 234 a which is provided over the third insulating film 210 a and at least partly overlaps with the second conductive film 216 a; a fifth conductive film 214 which is provided over the third insulating film 210 a and at least partly overlaps with the first oxide semiconductor film 206 a and the first conductive film 204 a; a fourth insulating film 220 a which is provided over the third insulating film 210 a and whose upper surface is level with upper surfaces of the fourth conductive film 234 a and the fifth conductive film 214; a fifth insulating film 210 b provided over the fourth conductive film 234 a, the fifth conductive film 214, and the fourth insulating film 220 a; a second oxide semiconductor film 206 b which is provided over the fifth insulating film 210 b and at least partly overlaps with the fifth conductive film 214; a sixth conductive film 216 b which is provided over the second oxide semiconductor film 206 b and at least partly overlap with the second conductive film 216 a; a seventh conductive film 226 b which is provided over the second oxide semiconductor film 206 b and at least partly overlap with the third conductive film 226 a; a sixth insulating film 212 b provided over the sixth conductive film 216 b, the seventh conductive film 226 b, and the second oxide semiconductor film 206 b; an eighth conductive film 234 b which is provided over the sixth insulating film 212 b and at least partly overlaps with the sixth conductive film 216 b; a ninth conductive film 204 b which is provided over the sixth insulating film 212 b and at least partly overlaps with the second oxide semiconductor film 206 b and the fifth conductive film 214; a seventh insulating film 220 b provided over the eighth conductive film 234 b, the ninth conductive film 204 b, and the sixth insulating film 212 b; and a tenth conductive film 260 provided over the seventh insulating film 220 b. Note that an opening reaching the third conductive film 226 a is formed in the seventh insulating film 220 b, the sixth insulating film 212 b, the seventh conductive film 226 b, the second oxide semiconductor film 206 b, the fifth insulating film 210 b, the fourth insulating film 220 a, and the third insulating film 210 a. The tenth conductive film 260 and the third conductive film 226 a are in contact with each other through the opening. Note that an eighth insulating film 225 having a planar upper surface is preferably provided over the tenth conductive film 260 and the seventh insulating film 220 b.

Here, the description of the substrate 101 can be referred to for the substrate 201. The description of the first insulating film 102 can be referred to for the first insulating film 202. The description of the first conductive film 104 a can be referred to for the first conductive film 204 a. The description of the second insulating film 112 a can be referred to for the second insulating film 212 a. The description of the first oxide semiconductor film 106 a can be referred to for the first oxide semiconductor film 206 a. The description of the second conductive film 116 a can be referred to for the second conductive film 216 a. The description of the third conductive film 126 a can be referred to for the third conductive film 226 a. The description of the third insulating film 110 a can be referred to for the third insulating film 210 a. The description of the fourth conductive film 114 can be referred to for the fifth conductive film 214. The description of the fourth insulating film 120 a can be referred to for the fourth insulating film 220 a. The description of the fifth insulating film 110 b can be referred to for the fifth insulating film 210 b. The description of the second oxide semiconductor film 106 b can be referred to for the second oxide semiconductor film 206 b. The description of the fifth conductive film 116 b can be referred to for the sixth conductive film 216 b. The description of the sixth conductive film 126 b can be referred to for the seventh conductive film 226 b. The description of the sixth insulating film 112 b can be referred to for the sixth insulating film 212 b. The description of the seventh conductive film 104 b can be referred to for the ninth conductive film 204 b. The description of the seventh insulating film 120 b can be referred to for the seventh insulating film 220 b.

The fourth conductive film 234 a is provided in the same layer as the fifth conductive film 214.

The eighth conductive film 234 b is provided in the same layer as the ninth conductive film 204 b.

The tenth conductive film 260 serves as a bit line of a memory. A side surface of the tenth conductive film 260 is in contact with the seventh conductive film 226 b. As the tenth conductive film 260, a film selected from the conductive films given as examples of the second conductive film 216 a may be used.

Further, the first conductive film 204 a and the ninth conductive film 204 b each serve as a word line of the memory.

Furthermore, the fifth conductive film 214 serves as a second gate electrode of a transistor using the first oxide semiconductor film 206 a and a transistor using the second oxide semiconductor film 206 b.

In FIG. 8, a circuit diagram of the semiconductor device formed as described above is illustrated. FIG. 8 is a circuit diagram of a memory in which one transistor is provided per capacitor.

When FIG. 7B and FIG. 8 are compared, the first conductive film 204 a at least partly serves as a first word line WL1; the fourth conductive film 234 a at least partly serves as a first capacitor line CL1; the fifth conductive film 214 at least partly serves as a back gate line BG; the eighth conductive film 234 b at least partly serves as a second capacitor line CL2; the ninth conductive film 204 b at least partly serves as a second word line WL2; and the tenth conductive film 260 at least partly serves as a bit line BL.

Further, a capacitor C1 in FIG. 8 corresponds to a capacitor 270 a in FIG. 7B. The capacitor 270 a includes at least part of the second conductive film 216, at least part of the third insulating film 210 a, and at least part of the fourth conductive film 234 a. A capacitor C2 in FIG. 8 corresponds to a capacitor 270 b in FIG. 7B. The capacitor 270 b includes at least part of the sixth conductive film 216 b, at least part of the sixth insulating film 212 b, and at least part of the eighth conductive film 234 b.

A transistor Tr1 in FIG. 8 corresponds to a transistor 271 a in FIG. 7B. The transistor 271 a includes at least part of the first conductive film 204 a, at least part of the first oxide semiconductor film 206 a, at least part of the second conductive film 216 a, at least part of the third conductive film 226 a, and at least part of the fifth conductive film 214. A transistor Tr2 in FIG. 8 corresponds to a transistor 271 b in FIG. 7B. The transistor 271 b includes at least part of the ninth conductive film 204 b; at least part of the second oxide semiconductor film 206 b; at least part of the sixth conductive film 216 b; at least part of the seventh conductive film 226 b; and at least part of the fifth conductive film 214.

A memory cell MC1 includes the transistor Tr1 and the capacitor C1. A memory cell MC2 includes the transistor Tr2 and the capacitor C2.

Note that the memory cell MC1 and the memory cell MC2 are provided so as to at least partly overlap with each other. Here, in FIG. 7A, the area of a memory cell 280 is 2F×4F, that is 8F². However, the memory cell MC1 and the memory cell MC2 are provided in the area. Accordingly, in the case where the memory cell MC1 and the memory cell MC2 completely overlap with each other, the area per memory cell can be regarded as 4F², which is a half of 8F².

Note that the semiconductor device illustrated in FIG. 7B includes the eighth insulating film 225 having a planar upper surface as an uppermost layer. Therefore, a plurality of structures similar to the structure of the semiconductor device in FIG. 7B can be provided so as to be stacked. In the case where two structures of the semiconductor device in FIG. 7B are stacked and four memory cells are provided so as to completely overlap with each other in an area of 8 F², the area per memory can be regarded as 2F², which is one-quarter of 8F².

As described above, when the plurality of structures of the semiconductor device in FIG. 7B are provided so as to be stacked, the area per memory cell can be reduced, so that a memory having large storage capacity per unit area can be manufactured. Further, the yield of the semiconductor device is not decreased even when the degree of integration of the semiconductor device is increased.

Note that although FIG. 8 is a circuit diagram showing part of the memory, the scale can be expanded, so that a memory cell array in which a plurality of memory cells MC1 and memory cells MC2 are arranged in the same plane can be obtained.

Methods for wiring and reading data to/from the memory cell array will be described below.

A method for writing data to the memory cell array will be described. Data writing is performed on every memory cell. Specifically, a potential of the word line in a row which is selected arbitrarily is set to VH (potential higher than the sum of the threshold voltage (Vth) of a transistor and VDD (power supply potential)), and potentials of word lines in the other rows are set to GND (or lower than GND). Next, a potential of a bit line in a column which is selected arbitrarily is set to VDD, and the bit lines in the other columns are set to a floating potential (float). As a result, VDD is stored in a capacitor which is in a memory cell in the selected row and which is connected to the bit line in the selected column. Then, the potential of the word line in the selected row is set to GND (or lower than GND), whereby data is stored in the memory cell. After that, data is sequentially written to other memory cells. This is the method for wiring data to the memory cell array.

Alternatively, data writing is performed per row. For example, to write data to the memory cells at theist row, the potential VH is applied to the word line in the first row, and then, the potential VDD is applied to the bit line at each column of the memory cell(s) to which data 1 is written, whereas the potential GND is applied to the bit line at each column of the memory cell(s) to which data 0 is written. Then, the potential of the word line is changed to GND (or a potential lower than GND), whereby data is held in the capacitor. This operation is performed row by row, whereby data can be written in all memory cells. This is the method for wiring data to the memory cell array.

Low off-state current of the transistor using an oxide semiconductor film, which is one embodiment of the present invention, enables data written to the memory cell in this manner to be stored for a long time.

Then, a method for reading data will be described. Data reading is performed on every memory cell. First, a potential of a bit line in a column which is selected arbitrarily is set to a predetermined potential (fixed potential). Next, a potential of a word line in a row which is selected arbitrarily is set to VH, whereby a potential corresponding to data written to the capacitor is given to the bit line which is selected. After that, the given potential is read by a sense amplifier (not illustrated). Note that data is lost at the same time data is read. However, the operation of the sense amplifier amplifies the potential, so that data is written to the memory cell again. After that, data is sequentially read from other memory cells.

Alternatively, data reading is performed per row. For example, in the case where data in the memory cells in the first row are read, potentials of all the bit lines are set to a predetermined potential (a fixed potential). Next, potentials of the word line in the first row are set to VH, so that the potential of each of the bit lines is changed depending on data. This operation is performed row by row, whereby data can be read out from all the memory cells. This is the method for reading data from the memory cell array.

Here, a method for manufacturing the semiconductor device illustrated in FIG. 7B will be described with reference to FIGS. 9A to 9D, FIGS. 10A to 10C, FIGS. 11A to 11C, FIGS. 12A and 12B, FIGS. 13A and 13B, FIGS. 14A and 14B, and FIG. 15.

First, the substrate 201 is prepared.

Next, the first insulating film 202 is formed over the substrate 201 (see FIG. 9A).

Then, a conductive film is formed and then processed to form the first conductive film 204 a (see FIG. 9B).

After that, the second insulating film 212 a is formed (see FIG. 9C).

Next, an oxide semiconductor film is formed and then processed, so that the first oxide semiconductor film 206 a is formed (see FIG. 9D).

Subsequently, a conductive film is formed and then processed, so that the second conductive film 216 a and the third conductive film 226 a are formed (see FIG. 10A).

Then, the third insulating film 210 a is formed (see FIG. 10B).

Next, a conductive film is formed and then processed, so that a conductive film 244 a and a conductive film 254 are formed (see FIG. 10C).

Next, an insulating film 250 a is formed (see FIG. 11A).

Next, the insulating film 250 a, the conductive film 244 a, and the conductive film 254 are etched from the upper surfaces by CMP or the like, whereby the fourth insulating film 220 a, the fourth conductive film 234 a, and the fifth conductive film 214, whose upper surfaces are level with one another, are formed (see FIG. 11B). Note that when the insulating film 250 a, the conductive film 244 a, and the conductive film 254 are formed to have a sufficient thickness in advance, which makes it easy to form the fourth insulating film 220 a, the fourth conductive film 234 a, and the fifth conductive film 214, whose upper surfaces are level with one another.

Next, the fifth insulating film 210 b is formed (see FIG. 11C).

Next, an oxide semiconductor film is formed and then processed, so that the second oxide semiconductor film 206 b is formed (see FIG. 12A).

Next, a conductive film is formed and then processed, so that the sixth conductive film 216 b and the seventh conductive film 226 b are formed (see FIG. 12B).

Next, the sixth insulating film 212 b is formed (see FIG. 13A).

Next, a conductive film is formed and then processed, so that the eighth conductive film 234 b and the ninth conductive film 204 b are formed (see FIG. 13B).

Next, the seventh insulating film 220 b is formed (see FIG. 14A).

Next, the seventh insulating film 220 b, the sixth insulating film 212 b, the seventh conductive film 226 b, the second oxide semiconductor film 206 b, the fifth insulating film 210 b, the fourth insulating film 220 a, and the third insulating film 210 a are each partly etched to form the opening through which the third conductive film 226 a is exposed (see FIG. 14B).

Next, a conductive film is formed and then processed, so that the tenth conductive film 260 is formed (see FIG. 15).

Next, the eighth insulating film 225 is formed. Thus, the semiconductor device illustrated in FIG. 7B is manufactured.

Note that in order to planarize a surface of the eighth insulating film 225, CMP or the like may be performed thereon.

Since the semiconductor device manufactured as described above has a planar upper surface, the plurality of structures similar to the structure of the semiconductor device can be provided so as to be stacked as exemplified in FIG. 17B, so that the degree of integration of the semiconductor device can be increased. Further, the number of the layers used for the second gate electrodes can be reduced, and thus stress can be prevented from being increased even when the plurality of structures similar to the structure of the semiconductor device are stacked. In other words, even when the degree of integration of the semiconductor device is increased, the yield of the semiconductor device is not decreased.

This embodiment can be implemented in appropriate combination with any of the other embodiments.

Embodiment 3

In this embodiment, a semiconductor device having a structure different from those of the semiconductor devices in Embodiments 1 and 2 will be described with reference to FIGS. 16A and 16B.

FIG. 16A is a top view of a semiconductor device of one embodiment of the present invention. A cross-sectional view taken along dashed-dotted line A-B in FIG. 16A is FIG. 16B. In FIG. 16A, a film which is not seen due to overlap, an insulating film, and the like, are omitted for simplicity.

A structure from the substrate 201 to the fifth insulating film 210 b in a transistor illustrated in FIG. 16B is the same as that in the transistor illustrated in FIG. 7B. Therefore, Embodiment 2 can be referred to for the description of the components from the substrate 201 to the fifth insulating film 210 b.

The semiconductor device illustrated in FIG. 16B includes the first conductive film 204 a provided over the first insulating film 202 which is provided over the substrate 201; the second insulating film 212 a provided over the first conductive film 204 a and the first insulating film 202; the first oxide semiconductor film 206 a which is provided over the second insulating film 212 a and at least partly overlaps with the first conductive film 204 a; the second conductive film 216 a and the third conductive film 226 a which are provided over the first oxide semiconductor film 206 a; the third insulating film 210 a provided over the second conductive film 216 a, the third conductive film 226 a, and the first oxide semiconductor film 206 a; the fourth conductive film 234 a which is provided over the third insulating film 210 a and at least partly overlaps with the second conductive film 216 a; the fifth conductive film 214 which is provided over the third insulating film 210 a and at least partly overlaps with the first oxide semiconductor film 206 a and the first conductive film 204 a; the fourth insulating film 220 a which is provided over the third insulating film 210 a and whose upper surface is level with upper surfaces of the fourth conductive film 234 a and the fifth conductive film 214; the fifth insulating film 210 b provided over the fourth conductive film 234 a, the fifth conductive film 214, and the fourth insulating film 220 a; a sixth conductive film 216 c which is provided over the fifth insulating film 210 b and at least partly overlap with the second conductive film 216 a; a seventh conductive film 226 c which is provided over the fifth insulating film 210 b and at least partly overlap with the third conductive film 226 a; a second oxide semiconductor film 206 c which is provided over the sixth conductive film 216 c, the seventh conductive film 226 c, and the fifth insulating film 210 b and which at least partly overlaps with the fifth conductive film 214; a sixth insulating film 212 c provided over the second oxide semiconductor film 206 c; an eighth conductive film 204 c which is provided over the sixth insulating film 212 c and at least partly overlap with the second oxide semiconductor film 206 c and the fifth conductive film 214; a seventh insulating film 220 c provided over the eighth conductive film 204 c and the sixth insulating film 212 c; and a ninth conductive film 261 provided over the seventh insulating film 220 c. Note that an opening reaching the third conductive film 226 a is formed in the seventh insulating film 220 c, the sixth insulating film 212 c, the seventh conductive film 226 c, the second oxide semiconductor film 206 c, the fifth insulating film 210 b, the fourth insulating film 220 a, and the third insulating film 210 a. The ninth conductive film 261 and the third conductive film 226 a are in contact with each other through the opening. Note that an eighth insulating film 226 having a planar upper surface is preferably provided over the ninth conductive film 261 and the seventh insulating film 220 c.

Here, the description of the sixth conductive film 216 b can be referred to for the sixth conductive film 216 c. The description of the seventh conductive film 226 b can be referred to for the seventh conductive film 226 c. The description of the second oxide semiconductor film 206 b can be referred to for the second oxide semiconductor film 206 c. The description of the sixth insulating film 212 b can be referred to for the sixth insulating film 212 c. The description of the ninth conductive film 204 b can be referred to for the eighth conductive film 204 c. The description of the seventh insulating film 220 b can be referred to for the seventh insulating film 220 c. The description of the tenth conductive film 260 can be referred to for the ninth conductive film 261. The description of the eighth insulating film 225 can be referred to for the eighth insulating film 226.

The capacitor 270 a in FIG. 16B corresponds to the capacitor C1 in FIG. 8. The capacitor 270 a includes at least part of the second conductive film 216 a, at least part of the third insulating film 210 a, and at least part of the fourth conductive film 234 a. Further, the capacitor C2 in FIG. 8 corresponds to a capacitor 270 c in FIG. 16B. The capacitor 270 c includes at least part of the sixth conductive film 216 c, at least part of the fifth insulating film 210 b, and at least part of the fourth conductive film 234 a. In other words, the semiconductor device has a structure in which the capacitor line CL2 is not provided and the capacitor C2 is connected to the capacitor line CL1 in FIG. 8.

Further, the transistor Tr1 in FIG. 8 corresponds to the transistor 271 a in FIG. 16B. The transistor 271 a includes at least part of the first conductive film 204 a, at least part of the first oxide semiconductor film 206 a, at least part of the second conductive film 216 a, at least part of the third conductive film 226 a, and at least part of the fifth conductive film 214. The transistor Tr2 in FIG. 8 corresponds to a transistor 271 c in FIG. 16B. The transistor 271 c includes at least part of the eighth conductive film 204 c, at least part of the second oxide semiconductor film 206 c, at least part of the sixth conductive film 216 c, at least part of the seventh conductive film 226 c, and at least part of the fifth conductive film 214.

In other words, the number of the conductive films in the semiconductor device in this embodiment can be reduced by one as compared to that in the semiconductor device in Embodiment 2. Note that the area of a memory cell 281 is the same as that of the memory cell 280.

The description in Embodiment 2 can be referred to for methods for writing and reading data to/from the memory cell.

Since the semiconductor device manufactured as described above has a planar upper surface, the plurality of structures similar to the structure of the semiconductor device can be provided so as to be stacked as exemplified in FIG. 17B. Accordingly, the degree of integration of the semiconductor device can be increased. Further, the number of the layers used for the second gate electrodes of the transistors each including gate electrodes above and below its oxide semiconductor layer can be made small, and thus stress can be prevented from being increased even when the plurality of structures similar to the structure of the semiconductor device are stacked. In other words, the yield of the semiconductor device is not decreased even when the degree of integration of the semiconductor device is increased.

This embodiment can be implemented in appropriate combination with any of the other embodiments.

Embodiment 4

In this embodiment, examples of electronic devices to which any of Embodiments 1 to 3 is applied will be described.

FIG. 18A illustrates a portable information terminal. The portable information terminal illustrated in FIG. 18A includes a housing 9300, a button 9301, a microphone 9302, a display portion 9303, a speaker 9304, and a camera 9305, and has a function as a mobile phone. One embodiment of the present invention can be applied to the display portion 9303 and the camera 9305. Although not illustrated, an embodiment of the present invention can also be applied to an arithmetic unit, a wireless circuit, or a memory circuit inside the main body.

FIG. 18B illustrates a digital still camera. The digital still camera illustrated in FIG. 18B includes a housing 9320, a button 9321, a microphone 9322, and a display portion 9323. One embodiment of the present invention can be applied to a memory circuit inside the digital still camera.

FIG. 18C illustrates a double-foldable portable information terminal. The double-foldable portable information terminal illustrated in FIG. 18C includes a housing 9630, a display portion 9631 a, a display portion 9631 b, a hinge 9633, and an operation switch 9638.

Part or whole of the display portion 9631 a and/or the display portion 9631 b can function as a touch panel. By touching an operation key displayed on the touch panel, a user can input data, for example.

By using a semiconductor device of one embodiment of the present invention, the performance of an electronic device can be improved and the reliability of the electronic device can be improved.

This embodiment can be implemented in appropriate combination with any of the other embodiments.

This application is based on Japanese Patent Application serial no. 2011-263867 filed with Japan Patent Office on Dec. 1, 2011, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: a first transistor including a first oxide semiconductor film, the first transistor comprising: a first gate electrode; and a back gate electrode; and a second transistor including a second oxide semiconductor film, the second transistor comprising: the back gate electrode; and a second gate electrode.
 2. The semiconductor device according to claim 1, wherein the back gate electrode at least partly overlaps with the first gate electrode, and wherein the second gate electrode at least partly overlaps with the back gate electrode.
 3. The semiconductor device according to claim 1, wherein a voltage applied to the first gate electrode and a voltage applied to the second gate electrode are higher than a voltage applied to the back gate electrode.
 4. The semiconductor device according to claim 1, wherein the first oxide semiconductor film includes c-axis aligned crystalline oxide semiconductor, and wherein the second oxide semiconductor film includes c-axis aligned crystalline oxide semiconductor.
 5. The semiconductor device according to claim 1, further comprising a first structure and a second structure at least partly stacked over the first structure, wherein each of the first structure and the second structure includes the first transistor and the second transistor.
 6. A semiconductor device comprising: a first word line; a second word line; a back gate line; a first transistor on an insulating surface, the first transistor comprises: a first conductive film electrically connected to the first word line; a first insulating film over the first conductive film; a first oxide semiconductor film over the first insulating film; a second conductive film at least partly in contact with the first oxide semiconductor film; a third conductive film at least partly in contact with the first oxide semiconductor film; a second insulating film over the first oxide semiconductor film, the second conductive film, and the third conductive film; and a fourth conductive film over the second insulating film, the fourth conductive film electrically connected to the back gate line; and a second transistor at least partly overlapping with the first transistor, the second transistor comprises: the fourth conductive film; a third insulating film over the fourth conductive film; a second oxide semiconductor film over the third insulating film; a fifth conductive film at least partly overlapping with the second oxide semiconductor film; a sixth conductive film at least partly overlapping with the second oxide semiconductor film; a fourth insulating film over the second oxide semiconductor film, the fifth conductive film, and the sixth conductive film; and a seventh conductive film over the fourth insulating film, the seventh conductive film electrically connected to the second word line, wherein the first oxide semiconductor film at least partly overlaps with the first conductive film and the fourth conductive film, and wherein the second oxide semiconductor film at least partly overlaps with the fourth conductive film and the seventh conductive film.
 7. The semiconductor device according to claim 6, wherein a voltage applied to the first word line and a voltage applied to the second word line are higher than a voltage applied to the back gate line.
 8. The semiconductor device according to claim 6, further comprising a first structure and a second structure at least partly stacked over the first structure, wherein each of the first structure and the second structure includes the first transistor and the second transistor.
 9. The semiconductor device according to claim 6, further comprising a fifth insulating film over the seventh conductive film and the fourth insulating film, wherein the fifth insulating film has a planer upper surface.
 10. The semiconductor device according to claim 6, wherein the first oxide semiconductor film includes c-axis aligned crystalline oxide semiconductor, and wherein the second oxide semiconductor film includes c-axis aligned crystalline oxide semiconductor.
 11. The semiconductor device according to claim 6, wherein the second insulating film is in contact with a side surface of the second conductive film, a side surface of the third conductive film, and a side surface of the first oxide semiconductor film, and wherein the fourth insulating film is in contact with a side surface of the fifth conductive film, a side surface of the sixth conductive film, and a side surface of the second oxide semiconductor film.
 12. A semiconductor device comprising: a first word line; a second word line; a back gate line; a first conductive film on an insulating surface, the first conductive film electrically connected to the first word line; a first insulating film over the first conductive film; a first oxide semiconductor film at least partly overlapping with the first conductive film with the first insulating film interposed therebetween; a second conductive film over the first oxide semiconductor film; a third conductive film over the first oxide semiconductor film; a second insulating film over the first oxide semiconductor film, the second conductive film, and the third conductive film; a fourth conductive film over the second insulating film, the fourth conductive film at least partly overlapping with the first oxide semiconductor film, the fourth conductive film electrically connected to the back gate line; a fifth conductive film over the second insulating film, the fifth conductive film at least partly overlapping with the second conductive film; a third insulating film over the second insulating film, the fourth conductive film, and the fifth conductive film; a second oxide semiconductor film over the third insulating film, the second oxide semiconductor film at least partly overlapping with the fourth conductive film; a sixth conductive film adjacent to the second oxide semiconductor film; a seventh conductive film adjacent to the second oxide semiconductor film; a fourth insulating film over the second oxide semiconductor film, the sixth conductive film, and the seventh conductive film; an eighth conductive film over the fourth insulating film, the eighth conductive film at least partly overlapping with the second oxide semiconductor film, the eighth conductive film electrically connected to the second word line; a fifth insulating film over the fourth insulating film and the eighth conductive film; a ninth conductive film over the fifth insulating film; and an opening in the second insulating film, the third insulating film, the second oxide semiconductor film, the seventh conductive film, the fourth insulating film, and the fifth insulating film, the opening reaching the third conductive film, wherein the ninth conductive film is in contact with the third conductive film through the opening.
 13. The semiconductor device according to claim 12, wherein a voltage applied to the first word line and a voltage applied to the second word line are higher than a voltage applied to the back gate line.
 14. The semiconductor device according to claim 12, further comprising a tenth conductive film between the fourth insulating film and the fifth insulating film, the tenth conductive film at least partly overlapping with the sixth conductive film.
 15. The semiconductor device according to claim 12, wherein each of the sixth conductive film and the seventh conductive film is located over the second oxide semiconductor film.
 16. The semiconductor device according to claim 12, wherein each of the sixth conductive film and the seventh conductive film is located under the second oxide semiconductor film.
 17. The semiconductor device according to claim 12, further comprising a sixth insulating film over the ninth conductive film, the sixth insulating film having a planer upper surface.
 18. The semiconductor device according to claim 12, wherein a part of the first conductive film, a part of the first insulating film, a part of the first oxide semiconductor film, a part of the second conductive film, a part of the third conductive film, a part of the second insulating film, and a part of the fourth conductive film are included in a first transistor, and wherein a part of the fourth conductive film, a part of the third insulating film, a part of the second oxide semiconductor film, a part of the sixth conductive film, a part of the seventh conductive film, and a part of the eighth conductive film are included in a second transistor.
 19. The semiconductor device according to claim 18, further comprising a first structure and a second structure at least partly stacked over the first structure, wherein each of the first structure and the second structure includes the first transistor and the second transistor.
 20. The semiconductor device according to claim 12, wherein the first oxide semiconductor film includes c-axis aligned crystalline oxide semiconductor, and wherein the second oxide semiconductor film includes c-axis aligned crystalline oxide semiconductor.
 21. The semiconductor device according to claim 12, wherein the second insulating film is in contact with a side surface of the second conductive film, a side surface of the third conductive film, and a side surface of the first oxide semiconductor film, and wherein the fourth insulating film is in contact with a side surface of the sixth conductive film, a side surface of the seventh conductive film, and a side surface of the second oxide semiconductor film. 